WO2012059767A1

WO2012059767A1 - Artificial planar conductor using nano engineered metal films

Info

Publication number: WO2012059767A1
Application number: PCT/GB2011/052144
Authority: WO
Inventors: Francisco Javier Vazquez Sanchez; Martin Shelley; Steven Nightingale
Original assignee: Cobham Cts Ltd
Priority date: 2010-11-04
Filing date: 2011-11-04
Publication date: 2012-05-10
Also published as: GB201018646D0; GB201307916D0; GB2499141A

Abstract

A conductor for an electromagnetic wave comprises a plurality of interconnected conducting layers spaced from each other in a first direction by insulating layers. Each of the conducting layers has a thickness in the first direction smaller than the skin depth of the conducting layer at the frequency of the electromagnetic wave to be conducted in use. A planar strip or ground plane may be made using the conductor.

Description

ARTIFICIAL PLANAR CONDUCTOR USING NANO ENGINEERED METAL FILMS

The present invention relates to artificial conductors and, in particular, to a planar conductor that reduces the skin effect.

Electrical DC and AC power can be transmitted over hundreds of kilometres using copper conductors with a relatively small loss. However, RF and microwave signals suffer large attenuation over even a few tens of meters. The reason for this difference is the so-called "skin effect" whereby electromagnetic waves attenuate quickly inside even good conductors and, as a result, induced currents are concentrated close to their outer surfaces. The average depth of the currents in the conductor is known as the "skin depth" and is frequency dependant. The skin depth in copper for example at a frequency of 60Hz is 8.5mm, however, the skin depth is only 2μηι at a frequency of 1 GHz.

This means that only a relatively smal fraction of the metal in a typical transmission line conductor is used to carry RF and microwave energy and the effective resistivity per meter at RF is many orders of magnitude higher than it is at DC. The attenuation (in dB/m) of an air filled transmission line is proportional to the resistivity of the conductors. Hence, if it were possible to use the full conductor cross-section at RF, the line attenuation could be reduced by many others of magnitude.

In order to overcome the skin effect for high power RF transmission lines, the so- called "Litz cable" was invented at the beginning of the 20^th century. This cable uses many individual insulated wire conductors grouped together and twisted so that they drive current from the skin to the inside of the conductor and then back out again, allowing all the wires to carry currents across the full cross-section (as shown in Figure 1 ). The Litz cable is currently being used for many high power RF applications up to 1 MHz. However, its application to higher frequencies becomes impractical since each strand should be smaller than the skin depth (only a few microns at VHF and sub micron at microwave frequencies) and the number of individual wires and associated isolating layers becomes huge when forming even a small diameter wire. The present invention seeks to provide a conductor which overcomes the disadvantages of known conductors used at higher frequencies.

According to the present invention there is provided a conductor for an electromagnetic wave, the conductor comprising a plurality of interconnected conducting layers spaced from each other in a first direction by insulating layers, wherein each of the conducting layers has a thickness in the first direction smaller than the skin depth of the conducting layer at the frequency of the electromagnetic wave to be conducted in use.

In preferred embodiments, the insulating layers are made of one of alumina or Si0₂ while the conducting layers are metallic, such as aluminium for example. The conducting layers may be embedded in a dielectric substrate to aid forming a planar strip. The dielectric substrate is preferably thin, having a thickness in the first direction between 1 and 100 μΐη, preferably between 2 and 50 μΐη.

The conducting layers may be interconnected using via holes or by edge plating of the dielectric substrate. The conducting layers may be interdigitated or interconnected in a ladder structure, for example.

The thickness of the conducting layers may be between 20 and 2000 nm, preferably between 500 and 1000 nm. The insulating layers may have a thickness in the first direction between 0.1 and 5 μιη, preferably between 0.75 and 1.25 μΐη.

The frequency of the electromagnetic wave is between 1 and 10 GHz.

A planar strip may be made of the conductor according to the present invention, which may be mounted for example mounted on a thin dielectric substrate. A ground plane may also be made from the conductor in accordance with the present invention. A combination of at least one ground plane, preferably together with a planar strip, can be used to produce a transmission line such as stripline, microstrip and parallel plate waveguide using suitable dielectric material to support the spacing of the artificial conductors. A power dividing or combining network can be made of such a transmission. A filter on above fabricated using such transmission lines.

Figures 1A and 1 B show conventional Litz cables conventional Litz cables useable up to 1 MHz;

Figure 2A shows a side view a four layer low loss conductor in accordance with the present invention;

Figures 2B-E shows top views of the four layer low loss conductor shown in Figure 2A;

Figure 3A shows a conventional suspended pipeline;

Figure 3B shows a low loss suspended stripline using a planar low loss transmission line;

Figure 4 shows a HFSS model of a 4 layer low loss conductor with a line width of 1 mm, a lattice at 2mm, and aluminium layer thickness of 1 μΐη; and

Figure 5 is a graph representing line attenuation per 100mm for a stripline. Figure 1A shows a round 'Type 2' conventional Litz cable comprising bundles B of twisted 'Type V wire together with optional outer insulation O. The outer insulation O may be of textile yarn, tape or extruded compounds. Figure 1 B shows a round 'Type 3' conventional Litz cable comprising bundles B of twisted 'Type 2 Litz' wire together with optional outer insulation O. Twisted Type 2 Litz are individually insulated with insulating material I.

An artificial planar conductor concept according to the invention is described herewith. The concept proposed is to use modern manufacturing techniques to develop a planar transmission line that reduces the skin effect. The objective is to reduce attenuation by a factor of 3-4 rather than by several orders of magnitude (as Litz cables do). However, even this reduced attenuation has significant practical advantages, for example creating printed transmission lines with transmission losses similar to waveguides. A planar artificial conductor may be formed by multiple conducting layers, typically made of metal, embedded in a supporting dielectric substrate, each of them thinner than the skin depth at the frequency of operation. The conducting layers may be arranged in a stack for example, such that they are interdigitated or interconnected in a ladder structure using via holes or edge plating of the supporting substrate as shown in Figure 2.

Figure 2A shows a side view a low loss conductor in accordance with the present invention comprising conducting four conducting layers 1 -4, which may be made for example from a metal such as Aluminium. Figures 2B-E shows top views of each of the four layers 1 -4 of the low loss conductor shown in Figure 2A. The conducting layers 1 -4 are be separated by insulating layers 5.

Essentially, a single metal track is replaced by multiple thin metallic films 1 -4 separated by thin dielectrics. The thin metallic layers may be made for example from aluminium, and can be produced as films by vapour deposition or other manufacturing techniques and have a typical thicknesses of 500-1 OOOnm (i.e. thinner than the skin depth at microwave frequencies). These films 1 -4 may be stacked and insulated by thin (typically Ι μηι), low loss substrate layers 5, typically, but not limited to, alumina or Si0₂. The conducting layers are interconnected to distribute currents evenly across all the conducting layers, using via holes or plated edges which are alternated to make the cross-over possible (see Figure 2).

To enable practical handling, the multi-layer structure will need to be supported on a separate carrier. A thin dielectric substrate, typically 25 - 50μηι thick, may be used to support the structure minimizing dielectric losses. However, if the structure is backed by metal, it can form a low loss ground.

The artificial conductor described above can be used to form an ultra-low loss transmission line operating at RF and microwave frequencies for example. A possible practical implementation of a suspended stripline transmission line is shown in Figure 3 below, where it is compared to a conventional suspended stripline construction. Figure 3A shows a conventional suspended pipeline, while Figure 3B shows a low loss suspended stripline using a planar low loss transmission line. The elements may be made of metal for example. The layer 1 1 may be a thin dielectric substrate, for example, polyester. The layers 12 represent the novel transmission line structure, which is shown in Figure 2. In both cases, the layers are supported with the correct separation using foam blocks.

An initial performance evaluation according of the present invention is outlined below. In order to evaluate the performance of this kind of conductor compared to a conventional solid metal transmission line, the central conductor of a suspended stripline transmission line has been replaced by the new structure in an Electromagnetic simulator, ANSOFT HFSS (Figure 4). The structure has been analysed from L- to X-band (1 -10GHz). Four films of 1 μΐη aluminium have been connected as in Figure 2. At X-band, where the skin depth is typically about 1 .3μιη; hence, the proposed design uses all 4μιη (4 x 1 μιη) of conductor to drive the currents, whereas, for the solid conductor, only 2.6μιη (2x skin depth) is used.

The FE simulation has been compared in Figure 5 to a conventional transmission line using an solid aluminium conductor 20 μηη thick. In the figure, the line 20 shows the performance using the four layer low loss conductor and the line 21 that for a 20μηι solid aluminium conductor. In both cases, the ground planes are assumed to be perfect electrical conductors.

The analysis suggests that, in X-Band (8GHz), the transmission loss (in dB/m) can be reduced to about 70% of that achieved using a solid centre conductor (0.7dB/m vs 1 .OdB/m). At lower frequencies, the solid conductor line becomes less lossy than the new type of line because the skin depth increases beyond the thickness of two layers of aluminium. At even higher frequencies, the loss improvement is even more significant. It should be noted that the new line is not subject to the skin effect and the loss is relatively frequency independent compared to the solid conductor one.

The attenuation (dB/m) improvement obtained (70% compared to the solid line at 8GHz) might seem modest; however, only a four layer design has been simulated. Theoretically, using an 8 layer design, the loss should reduce by a further 50%, since the cross-section of the conductors is doubled (i.e. to 35% of the solid line at 8GHz). If this benefit can be achieved, it will introduce a significant change in loss and power handling of transmission lines (i.e. transforming a 1 dB/m line loss into 0.35dB/m) that can make printed beamformers and filters feasible where only waveguide technology could be used previously. Also, it might be a way to reduce power dissipation issues inside high power ICs.

Claims

1. A conductor for an electromagnetic wave, the conductor comprising a plurality of interconnected conducting layers spaced from each other in a first direction by insulating layers, wherein each of the conducting layers has a thickness in the first direction smaller than the skin depth of the conducting layer at the frequency of the electromagnetic wave to be conducted in use.

A conductor according to claim 1 , wherein the insulating layers are made of one of alumina and Si0₂.

A conductor according to claim 2, wherein the conducting layers are embedded in a dielectric substrate.

A conductor according to claim 3, wherein the dielectric substrate has a thickness in the first direction between 1 and 100 μιη, preferably between 2 and 50 μΐη.

A conductor according to claim 1 to 4, wherein the conducting layers interconnected by via holes.

A conductor according to claim 3 or claim 4, wherein the conducting layers are interconnected by edge plating of the dielectric substrate.

A conductor according to any preceding claim, wherein the conducting layers are interdigitated.

8. A conductor according to any preceding claim, wherein the conducting layers are interconnected in a ladder structure.

9. A conductor according to any preceding claim, wherein the thickness of the conducting layers is between 20 and 2000 nm, preferably between 500 and 1000 nm.

10. A conductor according to any preceding claim, wherein the insulating layers have a thickness in the first direction between 0.1 and 5 μιη, preferably between 0.75 and 1.25 μΐη.

1 1. A conductor according to any preceding claim, wherein the frequency of the electromagnetic wave is between 1 and 10 GHz.

12. A planar strip comprising a conductor according to any preceding claim.

13. A ground plane comprising a conductor according to any preceding claim.

14. A transmission line comprising at least one of a planar strip according to claim 12 and a ground plane according to claim 13.

15. A transmission line according to claim 14, wherein the transmission line forms one of a stripline, a microstrip and a parallel plate waveguide.

16. A network comprising a transmission line according to claim 14 or claim 15.

17. A network according to claim 16, wherein the network is a power dividing network.

18. A network according to claim 16, wherein the network is a combining network.

19. A filter comprising a transmission line according to claim 14 or claim 15.